1. Field of the Invention
The present invention relates to spectrophotometry. In particular the present invention relates to probes used in making spectrophotometric measurements of fluids. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Spectrophotometry is widely used in industrial applications for measuring concentrations of chemical compounds. Usually a sample is obtained from one or more locations in the chemical processing system and analyzed in a laboratory.
Analysis of the sample can proceed in several ways. However, one way is to measure the absorption of light by the chemical sample at one distance and then repeat the measurement at a second, different distance. In the late 1800's, the fundamentals of concentration as a function of absorption and path length were laid down by Bier. Thus, it is well known that altering path length so more than one measurement can be taken of absorption can yield concentration data.
Several devices perform these measurements. See for example the spectral analysis apparatus described by LeFebre, et al. in U.S. Pat. No. 4,786,171. Lefebre, et al. provide a light source and optical fiber wave guides to shine light through a sample of a chemical and back to an analyzer. A servo mechanism or linear stepper motor changes the path length of light through the sample. A mirror or prism can be used to reflect light from one optical fiber to the other.
See also the apparatus of Ross described in U.S. Pat. No. 4,023,909 and that in German patent DE 3,339,950 A1 owned by Hartmann & Braun.
The difficulty arises in performing these measurements accurately and repeatably in situ in the chemical processing system itself. For in situ measurements, ruggedness, reliability and compactness are essential. If the chemicals of interest are corrosive or radioactive, seals must be effective in preventing leakage. There is a need for an in situ spectrophotometric probe that reliably provides accurate data in hostile chemical environments.